Add a new blackbar detection mode for subtitles

[Jerroder]

Feature request

What problem does this feature solve?

When watching a movie with blackbars, subtitles un-trigger the detection and top and bottom LEDs are turned of, except for white where the subtitles are.

What does the proposed API look like?

How should this be implemented in your opinion?

Instead of detecting blackbars based on the whole red area, the bottom zone should be split in the middle, something like the green area

Are you willing to work on this yourself?

Sure I can write a little C++, but testing is a bit more complicated since HyperHDR is running on my webOS TV.